I'm not one bit surprised that low-noise zeners made a difference in this one - that's some really skimpy filtering there. 100n in parallel isn't going to do much, that's roughly 3 orders of magnitude out. Space permitting, I would suggest maybe 100 µF @ 100-160V for C19/C21 and 220-470 µF @ 63 V for C20/C22. (Nothing much wrong with running some wires to a better spot either when leaving the original caps in.) It's a somewhat surprising lack of refinement, the Japanese had learned that lesson about a decade prior.
R64-67 may have to be swapped out for ones with higher power dissipation in order to avoid an untimately demise due to power-up current, depending on what they are now. The odd protection diode (1N4002 or similar) may not be the worst idea either... if so, in reverse across Q35/Q37 and/or in parallel to R64-67, again reverse-biased in normal operation.
JWB, I'm new to DIYaudio forums but not new to DIY audio by any stretch. Hahhh!
I just scored a KMA 160 in need of repair and would like to chat about my options. I also have a KSA 80 which I like quite a bit. I was thinking of converting the KMA 160 to a KSA 80 like you did. I am local to the SF Bay Area and would like to chat. I'll try to reach you off line.
I just scored a KMA 160 in need of repair and would like to chat about my options. I also have a KSA 80 which I like quite a bit. I was thinking of converting the KMA 160 to a KSA 80 like you did. I am local to the SF Bay Area and would like to chat. I'll try to reach you off line.
These amps are really simple. What's working, or not working, about the 160 you've got?
Converting the other KSA-80 to a 160 is simple, and it gives you a spare preamp board to put into the broken 160 if the problem with the 160 is in that board. You'll need to obtain or make a cable with the appropriate Molex connectors and pins, and suitable wire, to jumper the former left channel into the right channel for mono operation. Compare the 80 and the 160 side by side with the top covers off and it's super easy to see what needs to be done.
Use a dynamic transistor tester to check all the driver transistors on the driver board, and if any of them are blown, you'll need to check all six transistors oin the heatsinked power module that's connected to that driver board.
At this point, those amps being 30 years old now, you should perform the following maintenance:
Clean and use deoxit on all cables and connectors on the driver boards. Replace every electrolytic capacitor OTHER than the main reservoir caps which are computer grade and may last another 50 years. For the caps on the preamp board, use Vishay 10,000 hour rated parts. I got mine from Digikey.
Take every power stage heatsink assembly off the chassis and disassemble them. Test every transistor, replace any open emitter resistors with the same type (fusible safety resistor), and clean off all the old heat sink grease. Apply new grease and reassemble.
For a minor cosmetic upgrade I took off all the external stainless cap screws and replaced them with button head black oxide allen head scres.
My personal recommendation, IF you choose to convert the 80 to a 160, is to take out the extra XLR input socket that you won't be using and install a Neutrik Speakon connector in its place. I am a huge fan of Speakons and have a habit of retrofitting them to most every amp I own.
Converting the other KSA-80 to a 160 is simple, and it gives you a spare preamp board to put into the broken 160 if the problem with the 160 is in that board. You'll need to obtain or make a cable with the appropriate Molex connectors and pins, and suitable wire, to jumper the former left channel into the right channel for mono operation. Compare the 80 and the 160 side by side with the top covers off and it's super easy to see what needs to be done.
Use a dynamic transistor tester to check all the driver transistors on the driver board, and if any of them are blown, you'll need to check all six transistors oin the heatsinked power module that's connected to that driver board.
At this point, those amps being 30 years old now, you should perform the following maintenance:
Clean and use deoxit on all cables and connectors on the driver boards. Replace every electrolytic capacitor OTHER than the main reservoir caps which are computer grade and may last another 50 years. For the caps on the preamp board, use Vishay 10,000 hour rated parts. I got mine from Digikey.
Take every power stage heatsink assembly off the chassis and disassemble them. Test every transistor, replace any open emitter resistors with the same type (fusible safety resistor), and clean off all the old heat sink grease. Apply new grease and reassemble.
For a minor cosmetic upgrade I took off all the external stainless cap screws and replaced them with button head black oxide allen head scres.
My personal recommendation, IF you choose to convert the 80 to a 160, is to take out the extra XLR input socket that you won't be using and install a Neutrik Speakon connector in its place. I am a huge fan of Speakons and have a habit of retrofitting them to most every amp I own.
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